This invention relates to apparatus for separating a copy sheet from a moving photoconductive imaging surface of an electrophotographic copier following transfer of a developed image from the imaging surface to a copy sheet.
In electrophotographic copiers of the so-called plain paper type, an electrostatic image of an original is first formed and developed on a suitable photoconductive imaging surface, such as the surface of a cylindrical drum, and then transferred to a separate sheet of paper, usually by mechanical pressure, electrostatic force or both. Owing to various attractive or adhesive forces developed between the copy sheet and the imaging surface during image transfer, particularly where a liquid developer is used, the copy sheet does not separate readily from the imaging surface but must be assisted by some type of pickoff.
One type of pickoff apparatus of the prior art directs a relatively high-velocity jet of air or other gas against a sheet edge portion as it emerges from the transfer station. In theory, the high-velocity stream in the region overlying the edge portion reduces the pressure in that region, allowing the unreduced pressure of the air trapped in the region underlying the same sheet portion to lift the sheet. In practice, however, some of the air from the jet enters the region between the sheet and the imaging surface, reducing the pressure in that region as well so as to lessen, often significantly, the pressure differential actually achieved.
Another type of pickoff of the prior art uses a mechanical element such as a blade which rides on the imaging surface so as to intercept the leading edge of the copy sheet as it moves past the element. Since such an element continually rides on and thus abrades an image-bearing portion of the photoconductive surface, the useful life of the surface is undesirably reduced.